1. Field of the Invention
This invention relates generally to the field of digital circuit design and, more particularly, to the design of an RPM controller.
2. Description of the Related Art
Fans are often used to evacuate warm air from enclosures in which electronic systems are contained. For example, most computer systems include one or more cooling fans to aid in circulating the air inside the enclosures and for maintaining the temperature inside the enclosures within an acceptable range. The increased airflow provided by fans typically aids in eliminating waste heat that may otherwise build up and adversely affect system operation. Employing cooling fans is especially helpful in ensuring proper operation for certain central processing units (CPUs) with relatively high operating temperatures.
Control of fans in a system typically involves a fan control unit executing a fan control algorithm. A fan control algorithm may determine the method for controlling one or more fans that are configured to evacuate warm air from a system enclosure. For example, the fan control algorithm may specify that a fan's rotational speed should be increased or decreased dependent upon a detected temperature. Such control algorithms may also involve turning off a fan if the temperature is deemed cool enough to do so, or in certain systems, such as personal computers (PCs) for example, lowering the rotational speed of the fan and allowing the fan to continue running at a minimum rotational speed.
For detecting the temperature, a temperature sensor may provide to the fan control unit a signal indicative of the current temperature of a particular temperature zone in the electronic system. Often, fans used for CPU and/or computer system cooling have a three-wire interface with wires for power, ground, and a tachometer signal. Fan drive systems often use a signal generator that provides a Pulse Width Modulated (PWM) signal to drive an external circuit that controls the voltage between the power and ground interfaces of the fan, which in turn controls the speed of the fan. Signal generators that provide PWM signals are useful because they provide a digital control for the pulse width of a signal. The fan is typically powered only for the duration of the pulse. Between pulses power to the fan is turned off, although the fan is typically still spinning during this time. The duty cycle of the PWM pulse train currently being provided to the fan determines the fan's speed. Another typical way to control three-wire fans is to drive the fan by utilizing a high side Field Effect Transistor (FET), thereby controlling the DC voltage supplied to the fan. Generally, this provides an effective dynamic control range of 3V, which typically ranges from 5V down to around 2V. The lower limit voltage (2V) is still sufficient to power the fan circuitry, and valid tachometer signals may still be obtained from the fan.
Alternatively, some computer systems use fan control circuitry that features a 4-wire fan interface, where the fourth wire typically carries an additional control signal from the system to the fan. Thus, for fan drive systems that use PWM signal generators, in addition to the power, ground, and tachometer signal, a four-wire fan will typically have a PWM-drive input, which is used to control the speed of the fan. In such systems, instead of switching the power to the entire fan on and off, generally only the power to the drive coils is switched, making the tachometer information available continuously. Another advantage of 4-wire fans is that the fan speed can typically be controlled at speeds as low as 10% of the fan's full speed. Many PC desktop and workstation cooling fan solutions today use open loop 4-wire fan control methods, or are thermistor based, where a thermistor is integrated into the fan.
Typically when an open-loop four-wire cooling fan control method is used, two fan curves are specified. The first is generally a desired Temperature-versus-PWM curve, and the second is usually a PWM-versus-RPM (Revolutions Per Minute—an indication of rotational fan speed) curve. Many currently available fan control devices implement the Temperature-versus-PWM curve, and the cooling fans must generally follow the tightly specified PWM-versus-RPM curve. Open loop four-wire fan control systems thus have to rely on the tight fan specifications supplied by the fan manufacturer in order to achieve the desired fan RPM for a given PWM command.
In addition, in most cases, merely driving the fan with a prescribed duty cycle may not facilitate correcting for fan aging, pressure changes, and other conditions that might affect the performance of the fan over time. Most present day solutions address these issues using analog comparators and RC ramps to create a continuous function with varying duty cycle, to control either a PWM input to a fan, or the drive voltage applied externally to the fan. Therefore, alternative fan control methods may be preferred for driving the fan, while retaining digital control of the fan. For example, it may be desirable to provide closed loop RPM (revolutions per minute) control. When RPM control is used, however, the ability to control the RPM in a closed loop may require every operational point along the desired operational profile of a given fan, which usually requires that all operational points be stored (to be used by the controller), which may lead to excessive memory requirements. For example, in most systems, two Bytes may be required to store operating (control) data for each temperature point for which control of the fan is desired.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.